An embodiment of the invention relates to an assembly for mounting a radiation emitting device and a radiation emitting device having such an assembly. In particular, an embodiment of the invention is directed to a bearing assembly for the rotational mounting of a rotary anode of an X-ray emitting device and array emitting device equipped with such a bearing assembly. The assembly and the device can be used, for example, in a radiological apparatus and in particular, in the field of medical imaging.
A radiological apparatus generally comprises means for emitting radiation, such as a tube for emitting X-ray radiation, and means for directing the radiation, such as a collimator in order to form and define a beam of the X-ray radiation, means for receiving the beam and means for forming an image, such as a radiographic image receiver, and means for positioning supporting an assembly comprising of X-ray tube and the image receiver. The assembly is movable in space about one or more axes. An assembly and radiological apparatus is shown in EP-A-972 490.
An X-ray tube, mounted in a medical radiography apparatus, generally comprises a cathode and an anode, both enclosed in an evacuated sealed envelope, in order to provide electrical insulation between the two electrodes. When a high voltage supply is applied by a generator to the terminals of the cathode and of the anode, a so-called “anode” current is established in the circuit through the generator that produces the high supply voltage. The anode current passes through the space between the cathode and the anode in the form of an electron beam that is received by the anode over a small area forming a focus from which the X-rays are emitted. When the electron beam reaches the anode, the X-rays are emitted by the anode. Only a small percentage of the energy supplied by the electrons is converted into X-rays, the rest of the energy being converted into heat. In order to prevent too great an increase in the temperature of the focus, the focus is formed over an axisymmetric surface of the anode, and the anode is rotated about an axis of rotation. The portion of the axisymmetric surface of the anode forming the focus, located opposite the immovable cathode, continuously moves over the axisymmetric surface of the anode, allowing the heat to be distributed over the whole axisymmetric surface of the anode.
In order to obtain a radiographic image having high resolution, it is desirable to provide a small X-ray source. In other words, the focus must be small. The cathode is designed to obtain an electron beam converging on a small area of the anode forming the focus.
However, when using the X-ray tube, the focus moves with respect to an initial position. This movement is partly due to the high-speed rotation of the anode that is accompanied by vibration, leading to radial movement of the anode and altering the distance between the cathode and the anode. Moreover, the increase in temperature of the X-ray tube due to the dissipation of heat causes the various constituent elements of the X-ray tube to expand. This expansion may cause a clearance to appear between structural elements of the X-ray tube, which is accompanied by additional vibration of the anode.
The defect in position of the focus causes broadening of the apparent X-ray source, or loss of spatial resolution of the focus, thus decreasing the resolution of a radiographic image that can be obtained. Loss of spatial resolution limits the resolution of a picture obtained from the X-ray source.